Prop now deep enough in the water

[RandyCP]

I have a 1984 Basstracker 17' with a 1999 50HP Mercury 2 stroke. I recently purchased this boat and the engine runs great. The problem is the prop is not deep enough in the water. The engine is as low as it can go on the transom. If the engine is level, the result is engine revving too high and not getting anywhere or sucking air. I have to tilt the trim way back and down to keep it from sucking air. What options do I have here? Iv'e included a couple of pictures. One pic shows the engine at level and the other pic shows the position the engine has to be to keep from sucking air. Thank you.

 

[flyingscott]

Your lower unit height looks good. Is there anything in front of the motor that could interfere with the water to the propeller. Could be a spun hub on the prop.

 

[Scott Danforth]

im going to agree, something wrong with the prop or hub. same boat I grew up with (although with the original 84 motor), never had any problems with ventilation.....hitting a large rock, however never had a problem for the prop hooking up and making the boat fly.

 

[RandyCP]

Behind the prop is clear. Another note, WOT is only 4,800rpm at 29mph. It could be a prop/ hub problem. I'll take the prop off and take a look. The current prop is 10" with 15 pitch. Thanks

 

[flyingscott]

You need to scratch a line across the backside of the prop across the hub and metal housing, and go run it and see if the line moves.

 

[RandyCP]

I checked the prop and hub. They look fine.

 

[flyingscott]

I checked the prop and hub. They look fine.

Did you do post #5 you can't check them just by looking at them.

 

[RandyCP]

I'll try scratching a line. I'm going fishing Sat. And I'll check it out. Thanks

 

[RandyCP]

Yep, the hub was bad. Replaced it and all is good. Thanks for the tips.

 

[merc850]

Isn't 4800rpm a little on the low side?

 

[flyingscott]

Isn't 4800rpm a little on the low side?

If the hub was slipping the rpms can be pulled down.

 

[jimmbo]

One would think if the hub is slipping the rpms would be higher

 

[Texasmark]

gofast.com, BAM prop slip calculator,

50 hp 3 cyl looper,

1.64 gear box
4800 rpm
15P prop
Screw in wood 42 mph
Recorded actual 29 mph
Slip 31%

If your prop was slipping, your rpms would be up and they aren't according to your input. If in doubt , take it to the limit of the function......take all the pitch out of the prop. What would the engine do???????? Rev toward infinity rpms, just like it does in N with no load on the engine! According to your tach data you are lugging at 700 rpms below the published recommended rpm range of 5000 to 5500, upper limit which is where 2 strokers like to run....usually 2 strokers have low cubes (1 per hp or less), giving you low torque so to get HP you need rpms.......HP is (torque x rpms)/5252.

5500 is the max rpm for that engine

Assuming you were running 5500:

1.64
5500 (if your 4800 is a bad tach and this were your real rpm)
15P
Screw in wood 48
Assume 15% slip (which is reasonable on that boat....my last boat was that boat, different brand, different bottom, 90 hp slip running 12-15 area).
41 actual (or if your speedometer were off with your reading 29)

Assuming the engine was loading up with that prop and you dropped to a 12P

1.64
5500
12P
Screw in wood 38
15%
32 actual

Slipping hub unloads the engine allowing it to spool up in rpms. Slipping hub increases the calculated slip as Jimmbo stated. Your 31% is indicative of that in the first calculation. Fixing the hub should have reduced your slip number, pulled your rpms down more, while possibly increasing your speed.....whether or not depends on the torque of the engine under higher load at lower rpms. You didn't publish your data after the "fix".

 

[flyingscott]

You guys need to read the whole post. The hub was slipping and revving to high until he trimmed it all the way in to make it work at all. At that angle there was probably enough pressure on the prop to make the hub catch. And being trimmed all the way down takes down the RPM and speed down but that hub was still slipping. I have had a prop that did that took me forever to find it because I was losing RPMs and speed and would have never thought of the hub. I kept looking for something wrong with the motor. So no they don't always just let go and rev to high. When the hub slips like that you lose prop speed while the motor seems to be running a decent RPM.

 

[Texasmark]

You guys need to read the whole post. The hub was slipping and revving to high until he trimmed it all the way in to make it work at all. At that angle there was probably enough pressure on the prop to make the hub catch. And being trimmed all the way down takes down the RPM and speed down but that hub was still slipping. I have had a prop that did that took me forever to find it because I was losing RPMs and speed and would have never thought of the hub. I kept looking for something wrong with the motor. So no they don't always just let go and rev to high. When the hub slips like that you lose prop speed while the motor seems to be running a decent RPM.

Glad you kicked in on this. I was wondering about your answer considering your reputation. Had me spoofed. While you are here, where'd you get your "handle"? I started out with dad's 10 hp, '56 (I think) gold top (with dark green bottom) bail-a-matic and all and later had a '60's vintage white 22. I liked them but I had nothing to which I could make a comparison at the time.

 

[flyingscott]

Glad you kicked in on this. I was wondering about your answer considering your reputation. Had me spoofed. While you are here, where'd you get your "handle"? I started out with dad's 10 hp, '56 (I think) gold top (with dark green bottom) bail-a-matic and all and later had a '60's vintage white 22. I liked them but I had nothing to which I could make a comparison at the time.

I grew up with skiing on a McCulloch 75 hp on a starcraft 15' del-ray. I have always liked them and not a lot of people collect them and I like to be different.